- Email: [email protected]
CIPHER: coded imager and polarimeter for high-energy radiation
Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
CIPHER: coded imager and polarimeter for high-energy radiation E. Caroli *, J.B. Stephen , W. Dusi , G. Bertuccio, M. Sampietro ITeSRE/CNR, Via Gobetti 101, 40129 Bologna, Italy Dipartimento di Elettronica ed Informazione, Politecnico, P.zza L. Da Vinci 32, 20133 Milano, Italy
Abstract The CIPHER instrument is a hard X- and soft c-ray spectroscopic and polarimetric coded mask imager based on an array of cadmium telluride micro-spectrometers. The position-sensitive detector (PSD) will be arranged in 4 modules of 32;32 crystals, each of 2;2 mm cross section and 10 mm thickness giving a total active area of about 160 cm. The micro-spectrometer characteristics allow a wide operating range from &10 keV to 1 MeV, while the PSD is actively shielded by CsI crystals on the bottom in order to reduce background. The mask, based on a modi"ed uniformly redundant array (MURA) pattern, is four times the area of the PSD and is situated at about 100 cm from the CdTe array top surface. The CIPHER instrument is proposed for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium-size satellite survey mission and to perform an innovative measurement of the Crab polarisation level. The CIPHER's "eld of view allows the instrument to keep a single source within the "eld of view for a long observation period without requiring a precise pointing system. Herein we describe the instrument design, together with results obtained in our development studies, in particular on CdTe micro-spectrometers and the integrated front-end electronics. Furthermore, we present the expected operational performance in terms of image and spectral quality (angular and energy resolution) and polarimetric capabilities for an observation of the Crab nebula from balloon altitudes. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction To date, the study of compact X- and c-ray sources have been limited to their spectral characteristics and timing variability; however further observational parameters are required to determine the precise emission mechanisms and physical conditions of the objects. Polarisation is one of these additional parameters because almost all mecha-
* Corresponding author. Tel.: #39-051-6398-678; fax: #39051-6398-723. E-mail address: [email protected] (E. Caroli).
nisms that generate high-energy emission from astrophysical sources involve strong magnetic "elds and lead to the production of polarised photons. Even if polarisation measurements in lower-energy bands have been extremely useful [1], and although telescopes such as the COMPTEL instrument on the Compton Gamma Ray Observatory are theoretically able to perform polarisation measurements, the sensitivity is such that no actual measurements have been performed at energies greater than about 10 keV (for a review see Ref. [2]). For several years our group has studied the development of a compact telescopes suitable for hard X- and soft c-ray sky surveys, based on the use
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 2 8 3 - 7
526
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
of thick cadmium telluride position-sensitive spectrometers [3]. The design concept of these instruments allow their operation also in polarimetric mode with promising performance [4]. In the following section we describe a small payload (CIPHER: Coded Imager and Polarimeter for High-Energy Radiation) suitable for use as a stratospheric balloon-borne imager and polarimeter experiment. In particular, we give a "rst evaluation of the CIPHER ability to perform a polarisation measurement of the Crab pulsar emission in the 10}1000 keV energy range.
2. Instrument overview The CIPHER instrument is a coded mask telescope based on a CdTe position-sensitive detector operational from 10 to 1000 keV. The CIPHER design was conceived as a balloon-borne payload primarily intended to perform measurements of the polarisation level of strong hard X- and soft c-ray astrophysical sources (the target of the "rst #ight will be the Crab pulsar) and secondarily to assess and verify the performance of such an instrumental concept for a small/medium-size satellite high-energy survey mission [4]. CIPHER will be operated in photon-by-photon mode, requiring a star sensor in order to have real-time reconstruction of the telescope pointing direction, while the "eld of view allows the use of a low-cost and low-weight platform with only moderate pointing stability (&10 ) and coarse pointing accuracy (&13). Table 1 gives the CIPHER operational characteristics and required resources. In order to reduce the background rate on the CIPHER detector, a shield system is foreseen. This system is composed of two elements (Table 2): (a) an active veto shield of CsI(Tl) scintillating crystals in a well-shaped geometry around the detector and extending above the detector top surface in order to restrict the FOV to the telescope aperture; (b) a passive shield with a stopping power equivalent to 1 mm tungsten and shaped as a truncated pyramid that encloses the volume between the detector and the coded-mask and is mainly devoted to reducing the cosmic-ray di!use background that is the dominant component up &200 keV [5].
Table 1 The CIPHER payload operational characteristics Key CIPHER operational characteristics Energy range Field of view (FCFV) Angular resolution Point source location accuracy Timing accuracy Required #oating altitude
10}1000 keV 83;103 27 ;22 2 for a 5r detection 100 ls 1}3 mbar
CIPHER required resources Pointing accuracy Pointing stabilisation Telemetry On board data storage Length Footprint Power Mass
&13 10 64 kbits/s 250 MB 140 cm 60;70 cm 60 W 70 kg
Table 2 CIPHER veto shields main characteristics Characteristics
The passive shield tube
Material Thickness Number of units
Tungsten 0.3 mm 4 rectangular layers Mass (only material) 7 kg Opacity '90% at 100 keV Low-energy threshold N/A Readout devices N/A
The active veto shield CsI(Tl) 30 mm 4 lateral #1 bottom 23 kg '70% at 500 keV &30 keV 5 PM
2.1. The position-sensitive detector The position-sensitive detector (PSD) comprises four identical modules of 32;32 cadmium telluride micro-spectrometers. The basic sensitive unit is a crystal of 2;2;10 mm. These units are used in the con"guration in which the optical axis is orthogonal to the charge collecting "eld, and are assembled on thin (300 lm) ceramic plates in linear modules that contain 32 units (with a typical pitch of 2.1 mm) with their integrated analogue readout electronics and bias circuits. These linear modules are packed together (2.6 mm spacing is technically achievable) to form a 32;32 matrix module inside an aluminium case. Below the linear module, two
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530 Table 3 CIPHER position-sensitive characteristics
Table 4 Characteristics and performance of the ELBA ASICS
Active pixel size Geometrical pixel size Linear module Matrix module Matrix module number Detector-active area PSD geometrical area Detector thickness CdTe mass Linear module support Matrix module container Detector frame PSD overall height
Channel per ASIC Dynamic range Linearity Shaping type Shaping times per channel Peaking time Single power supply Power dissipation (PA#Shaper) Equivalent noise charge Equivalent energy resolution
2;2 mm 2.6;2.1 mm 32 CdTe micro-crystals 32;32 pixels 2;2 164 cm 18;15 cm"270 cm 10 mm 1 kg Al O (0.3 mm thick) Aluminium Aluminium 5.5 cm
layers are foreseen containing the hybrid frontend electronics (FEE) with multiplexer and ADCs for the 1024 channels. The matrix module is therefore a complete and independent detector that can be tested and calibrated separately. Finally, the matrix modules are integrated (for a total sensitive area of 164 cm with 4096 pixels) and supported by a metallic (Al) or carbon "bre grid that also provides the mechanical interface for the active veto shield. In Table 3 the main characteristics of the "nal PSD are reported. The requirement that the CIPHER detector be sensitive down to 10 keV implies the use of a lownoise front-end ampli"er, at or below 500 electrons rms. for a detector capacitance of about 1 pF. This performance should be reached with minimum power dissipation ((1 mW/channel) to avoid heating problems at balloon altitude due to the high degree of pixellisation of the detector. This may be achieved through the ELBA project that implements an innovative preampli"er scheme using BiCMOS technology [6] together with a double-"ltering technique [7] which allows a compensation to be made for trapping e!ects. Table 4 summarises the expected "nal performance and characteristics of the ELBA ASIC. 2.2. The coded mask The imaging capability of CIPHER is obtained through the use of a coded aperture mask. The mask is positioned 100 cm from the PSD surface,
527
8}16 10}1000 keV (0.3% (simulation) Unipolar semi-Gaussian 2 (200 ns and 2 ls) 1.5 ls #4 V 590 lW per channel 455 }e (rms) 4.5 keV (FWHM)
Table 5 The CIPHER coded mask characteritics Material Element size Element thickness Basic pattern Elements number Mask dimensions Mask weight Mask opacity Open fraction Mask-PSD dist.
Tungsten (19 g/cm) 6.3;7.8 cm (3;3 pixels) 0.5 mm MURA with 23;23 elements 45;45 elements (2;2 cycle) 28.4;35.1 cm 1 kg 99% at 100 keV; 12% at 500 keV 50% 100 cm
and the design is based on a Modi"ed Uniformly Redundant Array (MURA) basic pattern of 23;23 elements [8]. The lateral size of each mask element is 3 times the PSD pitch in each direction, so sampling the mask shadow at a frequency which preserves the image SNR [9]. The mask elements are made of 0.5 mm thick tungsten being almost totally opaque (95%) up to 150 keV. Above this energy the mask becomes increasingly transparent thus exposing a larger detector area for polarisation measurements (Fig. 2). In the pure imaging regime the mask element sizes dictate an angular resolution of 27 ;22 within a fully coded "eld of view (FCFV) of &83;103. The mask pattern is almost self-supporting because each element is joined to those adjacent. In this manner only a very light structure (e.g. carbon "bre honeycomb) is needed to support the elements and sti!en the mask, without a!ecting the transparency of the open elements at low energy. Table 5 summarises the main CIPHER mask characteristics.
528
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
3. The detector performance In the traditional use of CdTe detectors the photons are incident on one of the electrodes, thus limiting the absorption thickness and the operative range up to 200}300 keV. Several years ago, the TeSRE group proposed a geometry in which the charge collection "eld is orthogonal to the optical axis of the detector (PTF: planar transverse "eld) [10]. In this con"guration the charge collection distance is independent of the photon absorption thickness that can be increased up to 1}2 cm, i.e. suitable for higher-energy applications (up to the MeV band). The CdTe con"guration proposed for the realisation of the CIPHER PSD has been tested for a long time in the laboratory [11,12]: the
Fig. 2. The CIPHER exposed and e$cient area: the exposed area is the sensitive area exposed through the mask; the e$cient area is the sensitive area times the total PSD e$ciency. The blue line indicates the transition between CIPHER pure imaging (a) and polarimetry (b) modes.
achievable energy resolution ranges from 9% at 60 keV to &2% at 0.5 MeV. A pair of spectra obtained with 2;2;10 mm PTF micro-spectrometers are shown in Fig. 1. In order to evaluate the sensitivity to polarisation we have performed Monte Carlo simulation of a CIPHER module to estimate the total detection e$ciency and the e$ciency for scattered, and in particular double, events. These results are reported in Table 6. The simulation has shown that both total- and double-event e$ciency does not appreciably (&1%) vary with photon incidence angle within the "eld of view (up to 103). Using these data and the mask transparency as functions of energy we have evaluated both the exposed detector area through the coded aperture and detector e$cient area (Fig. 2). From this "gure it is possible to point out that the CIPHER design is optimised to be a pure spectroscopic imager below 150 keV, while above this energy the instrument becomes suitable for polarimetric measurements. Fig. 1. Measured spectra of radioactive sources obtained with 2;2;10 mm CdTe micro-spectrometers in the PTF con"guration. These measurements are obtained with a rise time selection of the signals: only signals with a short rise time (&25% of total events) are collected. We expect to obtain similar results without e$ciency losses by compensating the signals with double-shaping techniques.
4. The polarimetric capabilities The use of a highly pixellated detector such as the CIPHER PSD is ideal for use as a polarimeter.
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
529
Table 6 CIPHER total and double-event e$ciency. These results have been obtained with Monte Carlo simulation. The errors on values are less then 2% E (keV)
20
Total e$ciency 1.0 Double-event 0.0 e$ciency
50
100
200
500
1000
1.0 0.036
0.999 0.064
0.833 0.170
0.414 0.118
0.289 0.074
In order to investigate the e$ciency of this telescope design for polarimetric measurements we have calculated the response of the instrument to a 100% polarised beam of photons in order to obtain the polarimetric modulation factor Q [13]. This is obtained by integrating the formula for the Compton polarimetric di!erential cross section over the solid angles de"ned by the physical geometry of the detection plane. It is then de"ned as
Fig. 3. The calculated polarimetric Q factor for the TWISTER PSD as a function of energy. The Q factor measures the detector's ability to measure the asymmetry in the scattering angle that is expected for polarised radiation.
N !N Q" , N #N , where N and N are the count rates in orthogonal , , detectors in the X/Y plane. In Fig. 3 we show the Q-factor as a function of photon energy from 100 keV to 1 MeV, showing that this design is very competitive with other proposed instrumentation [2]. From the Q-factor it is possible to assess the observational signi"cance of a polarised source by calculating the uncertainty in Q, which by propagation of errors gives a signi"cance of Q Q(C np" " p(Q) ((1#Q) where C"(N #N ) is the total number of re , corded counts. In particular, this number is related to double-hit event e$ciency that for the proposed CIPHER PSD typically ranges from &4% to 17% in the 0.05}1.0 MeV energy band (Table 6). We have simulated a polarimetric observation of the CRAB pulsar, taking a typical balloon observation time of 10 s, a standard CRAB spectrum consisting of a power law of slope -2 and assuming, conservatively, that the pulsar emission is of the total emission and that it will be 10% polarised. It
Fig. 4. The signi"cance of a CIPHER detection of the polarisation of the Crab pulsar emission for an observation time of 10 s. We have assumed an intrinsic 10% polarisation level of the source.
can be seen from Fig. 4 that CIPHER will be able to obtain signi"cant results ('3r) between 70 and 600 keV under these conditions.
5. Conclusion This "rst evaluation of the achievable performance of the CIPHER telescope as a polarimeter is promising and therefore e!orts shall be made to
530
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
propose this balloon-borne payload for funding in the framework of a larger and international collaboration. In the immediate future we have planned tests with CdTe array prototypes, both to verify the achievable performance in terms of imaging and polarimetry and to tune some key parameters of the integrated front-end electronics under development.
References [1] T. Velusamy, Mon. Not. Roy. Astron. Soc. 212 (1985) 359. [2] F. Lei, A.J. Dean, G.L. Hills, Space Sci. Rev. 82 (1997) 309. [3] E. Caroli et al., Proceedings of the ESA Symposium on Photon Detectors for Space Instrumentation, ESTEC, Nordwijk, 10-12 November, 1992, NH, ESA SP-356, December 1992, p. 27.
[4] E. Caroli et al., Conference Records of the IEEE Nuclear Science Symposium, paper NS 21}07,1998; also IEEE Trans.Nucl.Sci., 1999. [5] N. Gehrels, Nucl. Instr. and Meth. A 313 (1992) 513. [6] G. Bertuccio et al., SPIE Proceeding Series of Conference 3768, Hard X-ray, Gamma-Ray and Neutron Detector Physics, 1999, to be published. [7] J. Lund et al., IEEE Trans. Nucl. Sci. NS-43 (1995) 1441. [8] S.R. Gottesman, E.E. Fenimore, Appl. Opt. 28 (1989) 4344. [9] E. Caroli et al., Space Sci. Rev. 45 (1987) 349. [10] F. Casali et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 598. [11] E. Caroli et al., Il Nuovo Cimento 16C (1993) 727. [12] W. Dusi et al., SPIE Proceeding Series of Conference 3768, Hard X-ray, Gamma-Ray and Neutron Detector Physics, 1999, to be published. [13] M. Su!ert et al., Physica 25 (1959) 659.
CIPHER: coded imager and polarimeter for high-energy radiation E. Caroli *, J.B. Stephen , W. Dusi , G. Bertuccio, M. Sampietro ITeSRE/CNR, Via Gobetti 101, 40129 Bologna, Italy Dipartimento di Elettronica ed Informazione, Politecnico, P.zza L. Da Vinci 32, 20133 Milano, Italy
Abstract The CIPHER instrument is a hard X- and soft c-ray spectroscopic and polarimetric coded mask imager based on an array of cadmium telluride micro-spectrometers. The position-sensitive detector (PSD) will be arranged in 4 modules of 32;32 crystals, each of 2;2 mm cross section and 10 mm thickness giving a total active area of about 160 cm. The micro-spectrometer characteristics allow a wide operating range from &10 keV to 1 MeV, while the PSD is actively shielded by CsI crystals on the bottom in order to reduce background. The mask, based on a modi"ed uniformly redundant array (MURA) pattern, is four times the area of the PSD and is situated at about 100 cm from the CdTe array top surface. The CIPHER instrument is proposed for a balloon experiment, both in order to assess the performance of such an instrumental concept for a small/medium-size satellite survey mission and to perform an innovative measurement of the Crab polarisation level. The CIPHER's "eld of view allows the instrument to keep a single source within the "eld of view for a long observation period without requiring a precise pointing system. Herein we describe the instrument design, together with results obtained in our development studies, in particular on CdTe micro-spectrometers and the integrated front-end electronics. Furthermore, we present the expected operational performance in terms of image and spectral quality (angular and energy resolution) and polarimetric capabilities for an observation of the Crab nebula from balloon altitudes. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction To date, the study of compact X- and c-ray sources have been limited to their spectral characteristics and timing variability; however further observational parameters are required to determine the precise emission mechanisms and physical conditions of the objects. Polarisation is one of these additional parameters because almost all mecha-
* Corresponding author. Tel.: #39-051-6398-678; fax: #39051-6398-723. E-mail address: [email protected] (E. Caroli).
nisms that generate high-energy emission from astrophysical sources involve strong magnetic "elds and lead to the production of polarised photons. Even if polarisation measurements in lower-energy bands have been extremely useful [1], and although telescopes such as the COMPTEL instrument on the Compton Gamma Ray Observatory are theoretically able to perform polarisation measurements, the sensitivity is such that no actual measurements have been performed at energies greater than about 10 keV (for a review see Ref. [2]). For several years our group has studied the development of a compact telescopes suitable for hard X- and soft c-ray sky surveys, based on the use
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 2 8 3 - 7
526
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
of thick cadmium telluride position-sensitive spectrometers [3]. The design concept of these instruments allow their operation also in polarimetric mode with promising performance [4]. In the following section we describe a small payload (CIPHER: Coded Imager and Polarimeter for High-Energy Radiation) suitable for use as a stratospheric balloon-borne imager and polarimeter experiment. In particular, we give a "rst evaluation of the CIPHER ability to perform a polarisation measurement of the Crab pulsar emission in the 10}1000 keV energy range.
2. Instrument overview The CIPHER instrument is a coded mask telescope based on a CdTe position-sensitive detector operational from 10 to 1000 keV. The CIPHER design was conceived as a balloon-borne payload primarily intended to perform measurements of the polarisation level of strong hard X- and soft c-ray astrophysical sources (the target of the "rst #ight will be the Crab pulsar) and secondarily to assess and verify the performance of such an instrumental concept for a small/medium-size satellite high-energy survey mission [4]. CIPHER will be operated in photon-by-photon mode, requiring a star sensor in order to have real-time reconstruction of the telescope pointing direction, while the "eld of view allows the use of a low-cost and low-weight platform with only moderate pointing stability (&10 ) and coarse pointing accuracy (&13). Table 1 gives the CIPHER operational characteristics and required resources. In order to reduce the background rate on the CIPHER detector, a shield system is foreseen. This system is composed of two elements (Table 2): (a) an active veto shield of CsI(Tl) scintillating crystals in a well-shaped geometry around the detector and extending above the detector top surface in order to restrict the FOV to the telescope aperture; (b) a passive shield with a stopping power equivalent to 1 mm tungsten and shaped as a truncated pyramid that encloses the volume between the detector and the coded-mask and is mainly devoted to reducing the cosmic-ray di!use background that is the dominant component up &200 keV [5].
Table 1 The CIPHER payload operational characteristics Key CIPHER operational characteristics Energy range Field of view (FCFV) Angular resolution Point source location accuracy Timing accuracy Required #oating altitude
10}1000 keV 83;103 27 ;22 2 for a 5r detection 100 ls 1}3 mbar
CIPHER required resources Pointing accuracy Pointing stabilisation Telemetry On board data storage Length Footprint Power Mass
&13 10 64 kbits/s 250 MB 140 cm 60;70 cm 60 W 70 kg
Table 2 CIPHER veto shields main characteristics Characteristics
The passive shield tube
Material Thickness Number of units
Tungsten 0.3 mm 4 rectangular layers Mass (only material) 7 kg Opacity '90% at 100 keV Low-energy threshold N/A Readout devices N/A
The active veto shield CsI(Tl) 30 mm 4 lateral #1 bottom 23 kg '70% at 500 keV &30 keV 5 PM
2.1. The position-sensitive detector The position-sensitive detector (PSD) comprises four identical modules of 32;32 cadmium telluride micro-spectrometers. The basic sensitive unit is a crystal of 2;2;10 mm. These units are used in the con"guration in which the optical axis is orthogonal to the charge collecting "eld, and are assembled on thin (300 lm) ceramic plates in linear modules that contain 32 units (with a typical pitch of 2.1 mm) with their integrated analogue readout electronics and bias circuits. These linear modules are packed together (2.6 mm spacing is technically achievable) to form a 32;32 matrix module inside an aluminium case. Below the linear module, two
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530 Table 3 CIPHER position-sensitive characteristics
Table 4 Characteristics and performance of the ELBA ASICS
Active pixel size Geometrical pixel size Linear module Matrix module Matrix module number Detector-active area PSD geometrical area Detector thickness CdTe mass Linear module support Matrix module container Detector frame PSD overall height
Channel per ASIC Dynamic range Linearity Shaping type Shaping times per channel Peaking time Single power supply Power dissipation (PA#Shaper) Equivalent noise charge Equivalent energy resolution
2;2 mm 2.6;2.1 mm 32 CdTe micro-crystals 32;32 pixels 2;2 164 cm 18;15 cm"270 cm 10 mm 1 kg Al O (0.3 mm thick) Aluminium Aluminium 5.5 cm
layers are foreseen containing the hybrid frontend electronics (FEE) with multiplexer and ADCs for the 1024 channels. The matrix module is therefore a complete and independent detector that can be tested and calibrated separately. Finally, the matrix modules are integrated (for a total sensitive area of 164 cm with 4096 pixels) and supported by a metallic (Al) or carbon "bre grid that also provides the mechanical interface for the active veto shield. In Table 3 the main characteristics of the "nal PSD are reported. The requirement that the CIPHER detector be sensitive down to 10 keV implies the use of a lownoise front-end ampli"er, at or below 500 electrons rms. for a detector capacitance of about 1 pF. This performance should be reached with minimum power dissipation ((1 mW/channel) to avoid heating problems at balloon altitude due to the high degree of pixellisation of the detector. This may be achieved through the ELBA project that implements an innovative preampli"er scheme using BiCMOS technology [6] together with a double-"ltering technique [7] which allows a compensation to be made for trapping e!ects. Table 4 summarises the expected "nal performance and characteristics of the ELBA ASIC. 2.2. The coded mask The imaging capability of CIPHER is obtained through the use of a coded aperture mask. The mask is positioned 100 cm from the PSD surface,
527
8}16 10}1000 keV (0.3% (simulation) Unipolar semi-Gaussian 2 (200 ns and 2 ls) 1.5 ls #4 V 590 lW per channel 455 }e (rms) 4.5 keV (FWHM)
Table 5 The CIPHER coded mask characteritics Material Element size Element thickness Basic pattern Elements number Mask dimensions Mask weight Mask opacity Open fraction Mask-PSD dist.
Tungsten (19 g/cm) 6.3;7.8 cm (3;3 pixels) 0.5 mm MURA with 23;23 elements 45;45 elements (2;2 cycle) 28.4;35.1 cm 1 kg 99% at 100 keV; 12% at 500 keV 50% 100 cm
and the design is based on a Modi"ed Uniformly Redundant Array (MURA) basic pattern of 23;23 elements [8]. The lateral size of each mask element is 3 times the PSD pitch in each direction, so sampling the mask shadow at a frequency which preserves the image SNR [9]. The mask elements are made of 0.5 mm thick tungsten being almost totally opaque (95%) up to 150 keV. Above this energy the mask becomes increasingly transparent thus exposing a larger detector area for polarisation measurements (Fig. 2). In the pure imaging regime the mask element sizes dictate an angular resolution of 27 ;22 within a fully coded "eld of view (FCFV) of &83;103. The mask pattern is almost self-supporting because each element is joined to those adjacent. In this manner only a very light structure (e.g. carbon "bre honeycomb) is needed to support the elements and sti!en the mask, without a!ecting the transparency of the open elements at low energy. Table 5 summarises the main CIPHER mask characteristics.
528
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
3. The detector performance In the traditional use of CdTe detectors the photons are incident on one of the electrodes, thus limiting the absorption thickness and the operative range up to 200}300 keV. Several years ago, the TeSRE group proposed a geometry in which the charge collection "eld is orthogonal to the optical axis of the detector (PTF: planar transverse "eld) [10]. In this con"guration the charge collection distance is independent of the photon absorption thickness that can be increased up to 1}2 cm, i.e. suitable for higher-energy applications (up to the MeV band). The CdTe con"guration proposed for the realisation of the CIPHER PSD has been tested for a long time in the laboratory [11,12]: the
Fig. 2. The CIPHER exposed and e$cient area: the exposed area is the sensitive area exposed through the mask; the e$cient area is the sensitive area times the total PSD e$ciency. The blue line indicates the transition between CIPHER pure imaging (a) and polarimetry (b) modes.
achievable energy resolution ranges from 9% at 60 keV to &2% at 0.5 MeV. A pair of spectra obtained with 2;2;10 mm PTF micro-spectrometers are shown in Fig. 1. In order to evaluate the sensitivity to polarisation we have performed Monte Carlo simulation of a CIPHER module to estimate the total detection e$ciency and the e$ciency for scattered, and in particular double, events. These results are reported in Table 6. The simulation has shown that both total- and double-event e$ciency does not appreciably (&1%) vary with photon incidence angle within the "eld of view (up to 103). Using these data and the mask transparency as functions of energy we have evaluated both the exposed detector area through the coded aperture and detector e$cient area (Fig. 2). From this "gure it is possible to point out that the CIPHER design is optimised to be a pure spectroscopic imager below 150 keV, while above this energy the instrument becomes suitable for polarimetric measurements. Fig. 1. Measured spectra of radioactive sources obtained with 2;2;10 mm CdTe micro-spectrometers in the PTF con"guration. These measurements are obtained with a rise time selection of the signals: only signals with a short rise time (&25% of total events) are collected. We expect to obtain similar results without e$ciency losses by compensating the signals with double-shaping techniques.
4. The polarimetric capabilities The use of a highly pixellated detector such as the CIPHER PSD is ideal for use as a polarimeter.
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
529
Table 6 CIPHER total and double-event e$ciency. These results have been obtained with Monte Carlo simulation. The errors on values are less then 2% E (keV)
20
Total e$ciency 1.0 Double-event 0.0 e$ciency
50
100
200
500
1000
1.0 0.036
0.999 0.064
0.833 0.170
0.414 0.118
0.289 0.074
In order to investigate the e$ciency of this telescope design for polarimetric measurements we have calculated the response of the instrument to a 100% polarised beam of photons in order to obtain the polarimetric modulation factor Q [13]. This is obtained by integrating the formula for the Compton polarimetric di!erential cross section over the solid angles de"ned by the physical geometry of the detection plane. It is then de"ned as
Fig. 3. The calculated polarimetric Q factor for the TWISTER PSD as a function of energy. The Q factor measures the detector's ability to measure the asymmetry in the scattering angle that is expected for polarised radiation.
N !N Q" , N #N , where N and N are the count rates in orthogonal , , detectors in the X/Y plane. In Fig. 3 we show the Q-factor as a function of photon energy from 100 keV to 1 MeV, showing that this design is very competitive with other proposed instrumentation [2]. From the Q-factor it is possible to assess the observational signi"cance of a polarised source by calculating the uncertainty in Q, which by propagation of errors gives a signi"cance of Q Q(C np" " p(Q) ((1#Q) where C"(N #N ) is the total number of re , corded counts. In particular, this number is related to double-hit event e$ciency that for the proposed CIPHER PSD typically ranges from &4% to 17% in the 0.05}1.0 MeV energy band (Table 6). We have simulated a polarimetric observation of the CRAB pulsar, taking a typical balloon observation time of 10 s, a standard CRAB spectrum consisting of a power law of slope -2 and assuming, conservatively, that the pulsar emission is of the total emission and that it will be 10% polarised. It
Fig. 4. The signi"cance of a CIPHER detection of the polarisation of the Crab pulsar emission for an observation time of 10 s. We have assumed an intrinsic 10% polarisation level of the source.
can be seen from Fig. 4 that CIPHER will be able to obtain signi"cant results ('3r) between 70 and 600 keV under these conditions.
5. Conclusion This "rst evaluation of the achievable performance of the CIPHER telescope as a polarimeter is promising and therefore e!orts shall be made to
530
E. Caroli et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 525}530
propose this balloon-borne payload for funding in the framework of a larger and international collaboration. In the immediate future we have planned tests with CdTe array prototypes, both to verify the achievable performance in terms of imaging and polarimetry and to tune some key parameters of the integrated front-end electronics under development.
References [1] T. Velusamy, Mon. Not. Roy. Astron. Soc. 212 (1985) 359. [2] F. Lei, A.J. Dean, G.L. Hills, Space Sci. Rev. 82 (1997) 309. [3] E. Caroli et al., Proceedings of the ESA Symposium on Photon Detectors for Space Instrumentation, ESTEC, Nordwijk, 10-12 November, 1992, NH, ESA SP-356, December 1992, p. 27.
[4] E. Caroli et al., Conference Records of the IEEE Nuclear Science Symposium, paper NS 21}07,1998; also IEEE Trans.Nucl.Sci., 1999. [5] N. Gehrels, Nucl. Instr. and Meth. A 313 (1992) 513. [6] G. Bertuccio et al., SPIE Proceeding Series of Conference 3768, Hard X-ray, Gamma-Ray and Neutron Detector Physics, 1999, to be published. [7] J. Lund et al., IEEE Trans. Nucl. Sci. NS-43 (1995) 1441. [8] S.R. Gottesman, E.E. Fenimore, Appl. Opt. 28 (1989) 4344. [9] E. Caroli et al., Space Sci. Rev. 45 (1987) 349. [10] F. Casali et al., IEEE Trans. Nucl. Sci. NS-39 (1992) 598. [11] E. Caroli et al., Il Nuovo Cimento 16C (1993) 727. [12] W. Dusi et al., SPIE Proceeding Series of Conference 3768, Hard X-ray, Gamma-Ray and Neutron Detector Physics, 1999, to be published. [13] M. Su!ert et al., Physica 25 (1959) 659.